1. Field of the Invention
The present invention relates to a display device, specifically to a structure of display device using a color forming material capable of repeatedly changing its color by a redox reaction; a reflective display; and an electronic paper.
2. Description of the Related Art
Recently, much research effort has been made to electronic paper as an electronic medium that is expected to displace paper. In contrast to conventional displays including CRTs and liquid crystal displays (LCDs), electronic paper requires the following characteristics: being a reflective display device; high white reflectivity and contrast ratio; high definition display; memory effect; low-voltage drive capability; slimness; lightness; and inexpensiveness, etc. In terms of its display characteristics, in particular, it is required that electronic paper has as excellent white reflectivity and contrast ratio as paper does, and the development of a display device offering all of these characteristics is far from easy. In addition, since a conventional display, paper medium, is generally a full-color display, colorization of electronic paper has strongly been demanded.
Techniques for electronic paper capable of displaying colors which have been so far proposed include a medium in which a color filter is formed on a reflective liquid crystal device, which has already been commercialized. The media, however, have the following problems: low light-utilizing efficiency due to the use of a polarizing plate; being incapable of displaying colors other than dark white; and low contrast ratio because of incapability of displaying black color.
As a bright reflective display device, there has been an electrophoretic device whose principle is to migrate charged white particles and charged black particles in an electric field. In such a device, however, it is practically difficult to completely reverse the white particles and the black particles so that it is hard to simultaneously satisfy high white reflectivity and high contrast ratio. Japanese Patent Application Laid-Open (JP-A) Nos. 2003-161964 and 2004-361514 disclose a reflective color display medium in which a color filter is formed on a electrophoretic device. However, it is clear that an excellent image quality cannot be obtained by forming a color filter on a display medium having low white reflectivity and low contrast ratio. JP-A Nos. 2004-520621 and 2004-536344 disclose an electrophoretic device which performs colorization by migrating particles having a plurality of colors. These devices cannot solve the above-mentioned problems in principle, and cannot simultaneously fulfill high white reflectivity and high contrast ratio.
A phenomenon in which electrochromic materials applied with a voltage show a reversible color change during the electrochemical redox reaction is called electrochromism. Electrochromic display device, which utilizes color change in electrochromic compounds that cause such a phenomenon, has emerged as a candidate for electrochromic paper because it serves as a reflective display device, has high white reflectivity as well as a memory effect, and can be driven at a low voltage. For example, JP-A Nos. 2001-510590, 2002-328401 and 2004-151265 disclose electrochromic devices in which an electrochromic compound is attached to the surface of semiconductor fine particles made of, for example, titanium oxide. Such electrochromic devices have effective structures because they can lower the amount of electric charge necessary for the drive, and they can accelerate coloration/decoloration reaction. However, the electrochromic compounds illustrated in the two publications form colors such as blue and green, but cannot form three primary colors, i.e., yellow, magenta and cyan, which are necessary for producing a full color display.
JP-A Nos. 62-71934 and 2006-71767 disclose aromatic dicarboxylic acid ester derivatives which are electrochromic compounds forming the three primary colors, i.e., yellow, magenta and cyan. Although these electrochromic compounds can form the three primary colors, they have no memory effect, so that when voltage-applying is stopped, these compounds lose their colors in about one second.
An electronic paper is supposed to be not used in the state of connecting continuously to a power source, but to be used with an internal battery. Accordingly, a display device of lower power consumption is demanded. If a display device has no memory performance, it needs continuous power supply for producing colors. But if a display device has memory performance, there is no need to supply power while it maintains to produce a color. Thus since the greater the memory performance, the lower the power consumption can be, so that enhancement of memory performance is important problem.